I don't understand why I need to put two lines in a specific order for it to work (recursion)

Question

I just started to learn about recursion.

This program is supposed to calculate n! and store the result into an output parameter.

void factorial(int n, int &f)
{
    if (n == 0)
    {
        f = 1;
    }
    else
    {
        factorial(n - 1, f);
        f *= n;
    }
}

int main()
{
    int f;
    factorial(5, f);
    cout << f;
}

This way it works perfectly. In my first attempt, the else condition looked like this

else
    {
    f *= n;
        factorial(n - 1, f);
    }

and the program didn't output anything but 1.

I really can't understand why it works in the first way but in the second it doesn't. Can you explain to me?

Answer 1

In your code, factorial(0, f) will set f = 1 . If you modify f before the call to factorial , it will be overwritten.

Note that this problem is not possible if you return int instead of taking int & as an argument.

Answer 2

For anyone wondering: Using @ForceBru's suggestion I ran it on a piece of paper and I got it. If I do it in the second way, the last thing called will be factorial(0) which will return 1, even if I already calculated to be n!.

Answer 3

f is getting over written to 1 in your last call f(0). If you switch the statements then f gets initialized to 1 in the last call f(0) followed by It's modification before return of each function call